In recent years, industrial products are more and more oriented toward energy saving from the viewpoints of, for example, environmental issues and soaring crude oil prices. This trend is also true with the field of construction and work vehicles that have so far mainly used hydraulic drive systems operated by diesel engines. Many of these vehicles are increasingly electrified to yield higher efficiency and consume less amounts of energy. For example, if the drive parts of the abovementioned construction and work vehicles are electrified, specifically, if electric motors are used for their drive sources, a number of energy-saving effects can be expected, such as driving of engines with high efficiency (for a hybrid model), improved power transmission efficiency, and recovery of regenerative electricity, in addition to reduced amounts of exhaust gases. In this field of construction and work vehicles, the electrification of forklift trucks is most advanced and what-is-called the “battery forklift trucks” that use electric power of a battery to drive a motor are put into practical use ahead of other types of vehicles and mainly compact forklift trucks are battery-powered. In even later years, following the battery forklift trucks, “hybrid vehicles” in which a diesel engine is combined with an electric motor begin to be available as commercial products in, for example, hydraulic excavators and engine-powered forklift trucks.
Among the construction and work vehicles that are being electrified in order to respond to environmental issues and energy-saving needs as described above, wheel loaders are expected to be a promising type of vehicle that, when hybridized, will yield a relatively significant effect of reduced fuel consumption. Conventional wheel loaders are a type of work vehicle that excavates and conveys, for example, earth and sand using a bucket portion of a work implement attached forwardly of the vehicle, while traveling along with power from an engine transmitted to its wheels via, for example, a torque converter and a transmission (T/M). Electrifying the Travel drive portion of such a wheel loader allows power transmission efficiency of the torque converter and transmission portion to be improved power transmission efficiency of electricity. In addition, the wheel loader repeatedly performs a traveling operation involving frequent starts and stops during working. If the travel drive portion is electrified, therefore, regenerative electric power during braking can be expected to be gained from the electric motor for traveling.
One known control method for a wheel loader hybrid system is disclosed in, for example, JP-2008-247269-A. The wheel loader disclosed in this document includes an engine and an electrical storage device as drive power sources. The wheel loader further includes a vehicular travel drive portion to which a hybrid drive system combining a motor generator, a planetary gear, and a transmission is applied. An engine speed is controlled based on an accelerator operation amount and an amount of charge of the electrical storage device, changeover of a shift speed of the transmission is controlled based on the accelerator operation amount, an operating condition of the motor generator, and a condition of charge of the electrical storage device, and torque to be generated in the engine and the motor generator is determined based on the accelerator operation amount, the travel speed, and the shift speed of the transmission.